Cloven foot

To show the cloven foot, i.e. to show a knavish intention, a
base motive. The allusion is to Satan, represented with the legs and
feet of a goat; and, however he might disguise himself, he could never
conceal his cloven feet. (See Bag O'Nails, Goat.)

“Real grief little influenced its composition and the cloven foot
peeps out in some letters written by him at the period.” —St.
James's Magazine